This invention relates to electronics packaging, more particularly to a method of conformally coating monolithic microwave integrated circuits (MMIC).
Integrated circuits used in micro/millimeter operating frequency applications are subject to attenuated performance (gain perturbation) when the circuits are subjected to moist environments. (As used herein, the term micro/millimeter frequencies refers to microwave and millimeter wave frequencies at or above 8 GHz.) Because of their moisture sensitivity, the integrated circuit chips used in these applications have traditionally required encapsulation o f to prevent moisture contamination of the circuit surfaces.
Accelerated aging test procedures such as EIA/JEDEC STANDARD, Steady State Temperature Humidity Bias Life Test, Test Method A101-B, are used to simulate the long-term reliability of non-hermetically sealed solid-state devices in humid environments. In the JEDEC test, the circuits are exposed, while in operation, to conditions of 85% relative humidity at a temperature of 85xc2x0 C. for a 1000 hour duration for the purpose of predicting their long-term reliability in humid environments (Mean Time to Failure or xe2x80x9cMTTFxe2x80x9d).
The difficulty with high frequency circuit encapsulation lies with the sensitivity of these types of circuits to most conformal coating/encapsulation materials. In general, coating materials applied in direct contact with the MMIC surface are applied in coating thicknesses of 0.005+/xe2x88x920.003 inches according to specifications such as MIL-I-46508. These relatively thick coatings in direct contact with the MMIC surfaces result in unacceptably high degrees of attenuated performance in high frequency applications. Therefore, in order to provide environmental resistance without the performance degradation inherent in conventional conformal coatings, traditionally, the MMIC circuitry is housed in a sealed container, usually made of metal or ceramic such that the MMIC operates in a hermetically sealed and isolated environment. Although effective, this packaging technique is expensive both in terms of component cost and assembly labor and, where gallium arsenide products are used, hermetic packaging may introduce hydrogen contamination from the nickel alloys used in the packaging.
Accordingly, what is needed is a method of applying a conformal coating to the surface of a MMIC chip in a substantially uniform, thin layer, such that the conformal coating is essentially invisible to the MMIC circuitry and, therefore, does not result in substantial attenuated performance.